Finding all PHP short tags

The best way to find short-tags in vim is to find all occurrences of <? not followed by a p:

/<?[^p]

The reason your regex is failing in vim is because /? finds literal question marks, while \? is a quantifier; /<\? in vim will attempt to find 0 or 1 less-than signs. This is backwards from what you might expect in most regular expression engines.

If you want to match short tags that are immediately followed by a new line, you cannot use [^p], which requires there to be something there to match which isn't a p. In this case, you can match "not p or end-of-line" with